First and foremost : progress on the growth levels (could be a bar, could be a bit of text like "1/5", anything so we can see where we are at basically)
Additional growth levels (most obvious suggestion, but I'm aware of the work it is on the image quantity side, so at your discretion ;) )
Height growth levels : the scrolls gives a full blown Giantess growth scene, but how about standard "growth" that would gradually lead to the character becoming a mini-Giantess (think like ... twice the average adult size or something ? keeping it "not too much" so the character can still evolve in society despite the growth) ?
I assume it's already planned, but some content for the slime girl ... (oh and a progress bar for her feeding too ?)
Aside from that, just "moar content" :D